Molybdenum disulfide is a well known lubricant. Unfortunately, its use as an additive in oils of lubricating viscosity is limited by its insolubility in oil. Consequently, oil-soluble molybdenum sulfur-containing compounds have been proposed and investigated for use as lubricating oil additives.
Commercially available dinuclear molybdenum sulfide lubricating oil additives are well known in the art. For example, the composition Mo.sub.2 O.sub.2 S.sub.2 (dtc).sub.2 can be added to a fresh oil of lubricating viscosity in order to enhance the oil's friction reducing properties. In the formula Mo.sub.2 O.sub.2 S.sub.2 (dtc).sub.2, dtc represents diorganodithiocarbamate ligands that are connected to the dinuclear molybdenum sulfide core.
As is known in the art, lubricating oil compositions containing molybdenum sulfide additives lose their friction reducing effectiveness over time when used in an internal combustion engine. While an enhancement in friction reducing properties is observed when the compound is added to a fresh oil, little if any friction reduction is observed when Mo.sub.2 O.sub.2 S.sub.2 (dtc).sub.2 is added to a used oil in which a loss in friction reducing properties has occurred, depending on the degree of oil aging and degradation.
Consequently, there remains a need for a composition that is capable of enhancing and restoring the friction reducing effectiveness of lubricating oils.